1. Field of the Invention
The present invention relates to a protective relay and more particularly to the one which makes decisions about operating conditions of an electric power system by the use of a digital computer thereby to protect equipment in the system.
2. Description of the Prior Art
FIG. 1 is an explanatory drawing of the principle of a protective relay indicated in Japanese Patent Laid-open No. 55-23779 (1980), wherein reference numeral 1 denotes rectifying and smoothing means for rectifying and smoothing electric current in a system, 2 denotes vector summation means for providing the vector sum of electric current values of the systems, 3 denotes scalar summation means for summing the above rectified and smoothed current values, 4 denotes rectifying and smoothing means for rectifying and smoothing the mentioned vector sum, 5 denotes comparison and decision means for comparing the mentioned vector sum (actuating quantity) and scalar sum (restraining quantity) to make a decision, and 6 denotes output means for outputting the result of the decision.
Now, the working principle of the apparatus of FIG. 1 will be described below with reference to the following operational expressions (1) and (2): ##EQU1## where I.sub.i.sup.t is a current value sampled at the time t and the subscript i represents a terminal number. And .SIGMA.I.sub.i.sup.t represents a vector sum, .parallel.I.sup.t .parallel. represents rectified and smoothed value, and ##EQU2## represents scalar sum, while K.sub.0, K.sub.1, and K.sub.2 are constants. Further, in the above example, the sampling frequency is arranged to be 12 times as large as the system frequency (30.degree. sampling).
Operations in FIG. 1 will be described below. All the sampled current values I.sub.i.sup.t of the system are rectified and smoothed by the rectifying and smoothing means 1 in the manner as indicated in the expression (2) and turned into .parallel.I.sub.i.sup.t .parallel. and the same are subjected to the scalar summing operation by the scalar summation means 3 and turned into ##EQU3## The above mentioned current values I.sub.i.sup.t are also subjected to the vector summing operation by the vector summation means 2 and, further, rectified and smoothed by the rectifying and smoothing means 4 to be turned into ##EQU4## In the comparison and decision means 5, the mentioned output ##EQU5## of the scalar summation means 3 is multiplied by an appropriate constant and the same and the above mentioned output ##EQU6## of the rectifying and smoothing means 4 together are subjected to the decision in accordance with the expression (1). If as the result the expression (1) holds true, an actuating signal is output from the comparison and decision means 5. The output means 6 provides an appropriate time delay for the mentioned signal and outputs the same as a final actuating signal.
With the above operational expression, it is intended to provide the differential characteristic as indicated in the solid line (B) in the operating characteristic curve of a general differential protective relay as shown in FIG. 2, wherein the minimum operating value is K.sub.0 and the ratio is K.sub.1 in the expression (1). Now, if instantaneous values only were rectified, there would be produced pulsating quantity and, hence, fluctuation in the operating characteristic depending on the sampling phase. Therefore, such rectifying and smoothing computation, for example, as indicated in the expression (2) must be made. By the computation according to the expression (2), it is intended to provide an effect substantially in the form of four-phase rectification and thereby to decrease the errors of the operating value and lessen the fluctuation of the differential characteristic depending on the sampling phase.
The prior art protective relay was structured as described above. Therefore, in such a case where the same was employed for protection of a bus-bar and therefore multiple-terminal data must be handled, a very long time was taken for computational processing of the expression (2). Thus, to avoid malfunction due to a CT saturation phenomenon or the like, it was required, for example, that values of the constants K.sub.0, K.sub.1 or the like were automatically varied depending on the quantity of the current so that the slope of the operating characteristic would be greatly varied in the larger current range. Thus, there were such problems that great restrictions were placed on the operating time as well as on the processing capacity of the computer.